1. Field of the Invention
The present invention relates generally to a magnetron for microwave ovens, and more particularly, to upper and lower shields fixedly attached to a top and bottom of a filament of a magnetron, respectively.
2. Description of the Related Art
Generally, a magnetron is constructed to have an anode and a cathode such that thermions are discharged from the cathode and spirally moved to the anode by an electromagnetic force. A spinning electron pole is generated around the cathode by the thermions and current is induced in an oscillation circuit of the anode, so that oscillation is continuously stimulated. An oscillation frequency of the magnetron is generally determined by the oscillation circuit, and has high efficiency and high output power. The magnetron is widely used in home appliances, such as microwave ovens, as well as industrial applications, such as high-frequency heating apparatuses, particle accelerators and radar systems.
The general construction and operation of the above-described magnetron are briefly described with reference to FIGS. 1 through 3.
As shown in FIG. 1, the magnetron generally includes a positive polar cylinder 101 made of an oxygen free copper pipe or the like, a plurality of vanes 102 disposed in the positive polar cylinder 101 to constitute a positive polar section along with the positive polar cylinder 101, and radially arranged at regular intervals to form a cavity resonator, and an antenna 103 connected to one of the vanes 102 to induce harmonics to an outside. The magnetron also includes a large-diameter strip ring 104 and a small-diameter strip ring 105 disposed on upper and lower portions of the vanes 102, respectively, to alternately and electrically connect the vanes 102 so that the vanes 102 alternately have the same electric potential as shown in FIG. 2.
Rectangular depressions 202 are formed in the vanes 102, respectively, to allow the strip rings 104 and 105 to alternately and electrically connect the vanes 102, and cause each opposite pair of the vanes 102 to be disposed in an inverted manner. According to the above-described construction, each of the pair of opposite vanes 102 and the positive polar cylinder 101 constitute a certain LC resonant circuit.
Additionally, a filament 106 in a form of a coil spring is disposed in an axial center portion of the positive polar cylinder 101, and an activating space 107 is provided between radially inside ends of the vanes 102 and the filament 106. An upper shield 108 and a lower shield 109 are attached to a top and bottom of the filament 106, respectively. A center lead 110 is fixedly welded to a bottom of the upper shield 108 while being passed through a through hole of the lower shield 109 and the filament 106. A side lead 111 is welded to a bottom of the lower shield 109. The center lead 110 and the side lead 111 are connected to terminals of an external power source (not shown), and therefore, forms a closed circuit in the magnetron.
An upper permanent magnet 112 and a lower permanent magnet 113 are provided to apply a magnetic field to the activating space 107 with opposite magnetic poles of the upper and lower permanent magnets 112 and 113 facing each other. An upper pole piece 117 and a lower pole piece 118 are provided to induce rotating magnetic flux generated by the permanent magnets 112 and 113 into the activating space 107. The above-described elements are enclosed in an upper yoke 114 and a lower yoke 115. Cooling fins 116 connect the positive polar cylinder 101 to the lower yoke 115, and radiate heat generated in the positive polar cylinder 101 to the outside through the lower yoke 115.
According to the above-described construction of the magnetron, when power is applied to the filament 106 from the external power source, the filament 106 is heated by operational current supplied to the filament 106, the thermions are emitted from the filament 106, and a group of thermions 301 are produced in the activating space 107 by the emitted thermions as shown in FIG. 3. The group of thermions 301 alternately imparts potential difference to each neighboring pair of the vanes 102 while being in contact with front ends of the vanes 102. The group of thermions 301 is rotated by an influence of the magnetic field formed in the activating space 107, and is moved from one state “i” to another state “f”. Accordingly, harmonics corresponding to a rotation speed of the thermion group 301 are generated by oscillation of the LC resonant circuit formed by the vanes 102 and the positive polar cylinder 101, and transmitted to the outside through the antenna 103.
Generally, a frequency is calculated by an equation       f    =          1              2        ⁢        π        ⁢                  LC                      ,where L is an inductance and C is a capacitance. Values of the variables of the above equation are determined by geometrical configurations of circuit elements. Thus, the configurations of the vanes 102 constituting part of the LC resonant circuit are principal factors in determining the frequency of harmonics.
Generally, electric and magnetic fields are generated in an activating space. A plurality of lines shown in the activating space 107 of FIG. 4 represent equipotential surfaces. The electric fields are always generated perpendicularly to the equipotential surfaces. Further, although not shown in FIG. 4, lines of a magnetic force are formed in the activating space 107 by the permanent magnets 112 and 113 disposed in upper and lower portions of the magnetron, respectively. In the magnetron, as a Lorentz force (F=q(E+vB) is exerted on the thermions generated from the filament 106 which functions as the cathode, and used to form the group of thermions 301 under the influence of the electric and magnetic fields in the activating space 107, the thermions are moved toward the vanes 102.
In the above equation, q represents an amount of electric charge, v represents a velocity of the electric charge, E represents an intensity of the electric field, and B represents an intensity of the magnetic field. The magnetic force always acts perpendicularly to a moving direction of the electric charge.
Some of the thermions that are applied with the exerted Lorentz force are moved around upper and lower portions of the filament 106. As shown in FIG. 1, the upper shield 108 has a shape of a hat and the lower shield 109 has a dented top surface. The thermions tend to escape from the activating space 107 due to the magnetic and electric fields formed in empty spaces between the upper shield 108 and the upper pole piece 117, and between the lower shield 109 and the lower pole piece 118, as shown in FIG. 4 (here, the lower shield and the lower pole piece are omitted in FIG. 4). Therefore, a phenomenon in which the thermions escape from the activating space 107 due to the Lorentz force causes an efficiency of the magnetron to decrease. In order to overcome the phenomenon, there has been used a method of mechanically preventing the escape of thermions by changing the geometrical configuration of the upper shield 108 (see FIG. 5A) in the shape of a hat, and changing a top surface of the lower shield 109 (see FIG. 5B) to be dented.
A diameter “A” of the upper shield 108 is 7.5 mm, an outer diameter “B” of an upper inclined portion 108a of the upper shield 108 is 6.7 mm, and a diameter “C” of a top portion 108b of the upper shield 108 is 5.35 mm. The upper shield 108 may be constructed within a certain error range. A diameter “D” of the lower shield 109 is 7.5 mm, an outer diameter “E” of the upper inclined part 109a of the lower shield 109 is 6.9 mm, a height“F” of the lower shield 109 is 2.5 mm, and a height “G” of the upper inclined part 109a of the lower shield 109 is 0.5 mm. The lower shield 109 may also be constructed within a certain error range. The conventional upper and lower shields 108 and 109 have relatively large sizes. Thus, the upper and lower shields 108 and 109 are positioned close to the upper and lower pole pieces 117 and 118 across an open space between the upper shield 108 and the upper pole piece 117, and another open space between the lower shield 109 and the lower pole piece 118. As a result, the conventional magnetron attempts to prevent thermions from escaping from an activating space by reducing open spaces through which thermions may escape from the activating space.
When distribution of the electromagnetic field is not uniform in the activating space 107 of the magnetron, electron beams are unstable and noise is emitted to the outside. In the magnetron using the upper and lower shields 108 and 109 shown in FIGS. 5A and 5B, a space charge distribution is typically asymmetrical around the upper and lower shields 108 and 109 in the activating space 107, as shown in FIG. 6. The asymmetry may cause a generation of very high harmonics in the magnetron, thus moving an axis of vanes upwardly and downwardly.
Further, it is ultimately electric and magnetic fields that apply force of a predetermined direction to thermions. Therefore, a suppression of using a mechanical configuration of the upper and lower shields 108 and 109, as shown in FIG. 5, is restrictive. Accordingly, the conventional magnetron is problematic in that it is impossible to fundamentally prevent the escape of thermions.